The field assembly 38 is provided with a field passage 214 for receiving the conduit 198, the conduit 198 being centered on the coil axis of 36 by a sleeve member 216, the sleeve member 216 being formed of a flexible material such as neoprene and having a tapered head portion 218 that extends external to the housing 22 for sealed clamping engagement by a clamping ring 220 that threadingly engages